was about barrel mirage. I was shooting a CZ 452 17HMR in 96* heat. There was only one relay so my barrel never got cooled down. The last course of fire was a 20 shot slowfire prone. After 10 shots I had 9 shots in the ten ring and one shot in the nine ring. Then my next 10 shots all grouped very nicely to the left about 2 inches. Instead of shooting a 198 or thereabouts I wound up with a very disappointing 186.